Nickel antimony titanium yellow pigments are by nature pale yellow pigments with high opacity. Overdyeing, i.e., over-coloring, with high quality organic pigments facilitates to obtain highly saturated full-tone colors covering the entire color spectrum, with the exception of blue and violet hues. This overcoloring results in a synergy between the relatively high opacity of the cost-efficient nickel antimony titanium yellow and the high color intensity of the organic overcoloring pigments, which are generally quite expensive.
This effect can also be obtained using titanium white; however, overcoloring always leads to greater brightening, that is, to less saturation, due to the high whitening power of titanium white pigments. Another disadvantage of titanium white overcoloring is the photocatalytic effect of titanium white pigments, resulting in a sharp decrease of light and weather-fastness of the expensive organic colorants. Consequently, fully saturated hues based on titanium white pigment “age” some four-times faster than mixtures of the same organic color components with nickel antimony titanium yellow.
In the past, this decisive use of nickel antimony titanium yellow was rarely used because the nickel titanium pigments currently available on the market are abrasive (grain-hard and sharp-edged), have poor gloss, and are inferior in terms of hiding power compared to titanium white. Moreover, the following economic background may be considered:
Nickel antimony titanium yellows account for a relatively small market shares among titanium pigments, as the following figures demonstrate: titanium white world market: 4,000,000 tons, titanium yellow world market: 20,000 tons, of which chromium antimony titanium yellow: 16,000 tons, of which nickel antimony titanium yellow: 4,000 tons.
The annual tonnage of chromium antimony titanium yellow is disproportionate to that of nickel antimony titanium yellow. As colored pigments, nickel antimony titanium yellows themselves do not provide for a satisfactory option to replace 100,000 annual tons of lead sulfochromate yellow and molybdate red pigments pursuant to the hazardous materials laws and environmental protection laws that have become increasingly stringent since 1980. The reason for this is the deficiencies, considered unchangeable, that are manifested in particular by the insufficient opacity, compared to titanium white and to chromium and cadmium yellows and by inadequate gloss and high abrasiveness.
These three deficiencies are the result of one and the same cause, specifically a mean particle size, primarily of nickel antimony titanium yellow pigments, that is too large at an average of 1000-2000 nm in the best qualities found on the market, while the optimum opacity of a pigmentary coloring agent is attained with a particle size of 300 nm and with optimized grain shape and surface. A pigment loses about 20% of its opacity when the mean particle diameter exceeds 500 nm. Finer commercial types according to the prior art are regularly slightly doped and greatly whitened. In addition due, to their high grinding costs expensive, highly doped, highly-fired products are unknown because they are inconsistent in terms of coloristic assessment.
Surface enlargement below a mean particle diameter of 300 nm should be avoided for color pigments because they become transparent when their size drops below a particle diameter of about half the wavelength of the light reflected by them, which is undesirable for applications of nickel antimony titanium yellow pigments. As the hardness of the particles is high, abrasiveness increases for spiky and sharp-edged particles. The object is therefore also to produce isometric particles (rhombi) that are chamfered or have beveled flattened or rounded corner zones. This cannot be achieved by coating the particles in accordance with DE-A-2 936 746 that acts in cooperation with surfactant agents like a slip agent and which prior art additionally teaches a separate processing step and use of auxiliary agents as the subject matter of the invention.
Nevertheless, as a further object set herein for avoiding color effects under different types of illumination (metamerism), a relatively smooth surface and also approximate uniformity of the (projected) edge lengths should be obtained; a spherical shape is unattainable under any conditions, however.
Taking the aforesaid into account, the relevant prior art shall be addressed:
In the past, a mean particle size of 300 nm has not been reached by any manufacturer with satisfactory results for highly doped and/or fully annealed nickel or chromium antimony rutile yellow pigments (TiO2<87%). Patent DE-A-3 202 158 describes in particular chromium antimony titanium yellows. In fact, the small dopings described therein inter alia with antimony and chromium at low firing temperatures<1000° C. with subsequent wet grinding in bead mills lead to a pigment with a narrow particle size distribution and corresponding fineness, sometimes also due to the softer grain of the mixed phase oxide pigments described therein. However, there are limits for hue control depending on how the reactive iron content will assume uncontrollable amounts if a non-metallic mill with resistant lining is not used as it is inventively in this case. Still, with a product according to the prior art in DE-A-3 202 158, increased photoactivity, and with appropriate very fine grinding, a high degree of whitening must be accepted. If iron abrasion is permitted in the milling process, the material will gray and results in interferences in PVC-based matrices. This is true even for mixed phase rutile-based oxide pigments that contain iron bound as a non-reactive component in the crystal lattice of the rutile, as in example 3 of DE-A-3 202 158.
In accordance with the teaching of DE-A-3 202 158, a coloristically favorable particle size distribution is attained when low doping, relatively low firing temperature, and wet milling are combined. However, this quoted application does not provide any information on particle size distribution and does not specify the type of milling precisely.
In the case of titanium dioxide, synthesis by the chloride process with the adjustment of the TiCl4 burner and blending-in agglomeration-preventing sand during the subsequent cooling and conveying process has already found a practical path for adjusting optimized particle size distributions (d50=approx. 280 nm for light of 550 nm wavelength) (see inter alia: Winkler, J: “Titanium Dioxide”, Hannover: Vincentz, 2003; ISBN 3-87870-148-9; pp. 35-37; 51-58). Although in this method small quantities of aluminum chloride are metered to the titanium tetrachloride for “rutilization”, it being unresolved how many lattice places in the rutile are really occupied by aluminum ions, this method is not promising e.g. for application of antimony and nickel chlorides to titanium tetrachloride upstream of the burner. Separation and inhomogeneous volatility of metal chlorides and metal oxychlorides prior to the lattice insertion of the metal ions is observed. A lengthy subsequent calcination period leads to reagglomeration.
The Ishihara Company is particularly active in the prior art. This is demonstrated by worldwide patent applications. These include for instance EP 1 245 646 (A1, corresponds to U.S. Pat. No. 6,576,052), in which a fine-particle TiO2 obtained from the chloride process, already 100 to 400 nm mean particle size, is re-ground to a corresponding fine primary particle size during a siloxane post-treatment and coating with aluminum phosphate using a jet mill. According to EP-A-1 273 555 (corresponds to U.S. Pat. No. 6,616,746), the same grain fineness of the raw pigment for coating is used with polyhydric alcohols and hydrolyzed amino silanes and/or aluminum hydroxide. The procedure is the same as the foregoing. Good dispersibility of the photostabilized products are claimed. For grinding, the pigment is comminuted after or during addition of the coating and stabilization reagents at a temperature of 120 to 300° C. in a jet mill or a similar “fluid energy mill” that permits a hydrolysis reaction of the amino siloxanes and other reactive components and prevents any reagglomeration during the coating process. This patent relates only to TiO2 in rutile modification (which is preferably formed by the additions of aluminum). After working up the batch, the coating is principally to act in a manner that prevents agglomeration and is photostabilizing, i.e. lastingly moderates the photocatalytic effects of the pigment.
Wet grinding of rutiles, that is also rutile yellow pigments, in high-intensity bead mills is prior art e.g. in accordance with DE-A-3 930 098. These are sold by a number of different specialty companies.
The option provided e.g. in DE-A-4 106 003 to obtain an a priori finer grain structure and thus save a grinding process by “alloying” the firing batch for a rutile brown pigment with small quantities of cerium, inter alia, cannot be performed with chromium and nickel titanium pigments due to the brighter hues that are more sensitive to fluctuations in doping.
Basically many companies seem to prefer wet precipitation of a precursor to the actual “hot” synthesis of the pigment which precursor is imprinted with the grain size and structure distribution and thus the fineness, which is maintained at this predetermined level until after calcination and final fine grinding. The hydroxyl groups on the surface of the freshly precipitated oxides and hydroxides represent a good promoter for the diffusive penetration of the rutile lattice with its numerous vacancies with foreign metal ions after evaporating the water above 150° C. However, the finer grain sizes possible due to the lower firing temperature are not yielding reproducible color intensity. Diameters still fluctuate between 800 and 1200 nm.
Proceeding from the prior art described in the foregoing, the object of the invention was to obtain a fine-particle, brilliant rutile-based pigment that is distinguished by superior opacity, gloss, and lower abrasiveness. Moreover, it should have the smallest possible or no iron content, for instance in the low ppm range in any case. Moreover, the invention should provide a method with which such a pigment can be produced in a particularly economical manner.